North Korea’s paranoid leader Kim Jong-un is desperately attempting to stem the flow of defectors – and state secrets – from North Korea into neighbouring China.

Earlier this week it was revealed Kim is now rotating guards along the border with China every half an hour in a bid to prevent complacency or defection plots.

And now sources within the hermit have warned new radio-tracking towers have been constructed along the border in order to eavesdrop on North Koreans with blackmarket Chinese mobile phones.

These phones are smuggled into North Korea from China and can be used in areas where mobile phone reception overlaps near the border.

They allow North Koreans to keep in touch with defectors who successfully fled the country, along with smugglers in China – but new security infrastructure is putting these phone owners at risk of detection.

Older, less accurate German-made locators could essentially be ‘shaken off’ by phone users, who merely had to walk to a different location every five minutes or so to lose the tracker.

But the new system can follow phone owners wherever they go, making every call a huge risk.

One source told Radio Free Asia (RFA): “The State Security Department recently ordered installations of the newest version of additional radio locators along the border with China in Yanggang province.

“They are known to perform better than German radio locators that are already installed, so those who use illegal phone users are filled with terror about them.”

And it is not just everyday citizens using illegal Chinese phones to make calls across the border, the source said. Business owners and company executives also ring abroad, as do members of North Korea’s high-society.

And the country’s booming black market, which is often the only way to secure enough food to survive, also benefits from the phones – something which means Kim will never launch a full crackdown and merely pick off unlucky individuals to make an example of.

The source said: “Illegal phone calls in the border area continue, despite that those making them could face hard labor in a camp in this extremely cold weather.

“Though the devices are the newest radio locators, it won’t be easy to uncover all illegal phone calls.

“If North Korea blocks all the illegal calls that executives, traders, and smugglers make, its market will be paralysed.”